Circuit breakers provide protection for conductors and loads. When the circuit breaker is closed and current is flowing through the circuit breaker, the circuit breaker provides thermal protection by monitoring the current passing through the circuit breaker to a load being protected by the circuit breaker and simulating the heating of the circuit breaker or load. If the primary conductors become too hot, their insulation will melt and its insulating properties will be compromised. Under such circumstances, the load protected by the circuit breaker can also be damaged by prolonged exposure to excessive heat.
A traditional thermal-magnetic circuit breaker provides thermal protection by passing current through a bimetal, which deflects as a function of temperature. When current passes through the bimetal, the heat generated in the bimetal models that of the primary conductors, which are also carrying current. When the heating of the bimetal becomes too great, the bimetal opens the circuit in the circuit breaker therefore interrupting the current flow. Without current passing through the circuit breaker, the circuit breaker begins to cool. However, mechanical circuit breakers using bimetal mechanisms typically remain at least partially deflected until the bimetal completely cools.
Electronic circuit breakers provide thermal protection by modeling the temperature of a conductor. Typically, electronic circuit breakers include a microcontroller that is operatively powered by the primary current passing through a primary conductor. The microcontroller is programmed to thermally model a primary conductor by measuring the potential created in a burden resistor when secondary current passes through that resistor. When the circuit breaker is tripped, the primary current in the primary conductor stops flowing and the secondary current drops to zero. Yet, the primary conductors remain at an elevated temperature until sufficient time has passed to allow the primary conductor to cool. For example, if primary current starts to flow again within approximately fifteen minutes, significant residual heat remains in the primary conductors and, thus, the conductors are still at an elevated temperature. This is problematic because after the circuit breaker is tripped and the primary current ceases, the microcontroller is no longer powered and cannot continue to thermally model the primary conductor as it cools. If the circuit breaker is powered up when residual heat remains in the primary conductors, the circuit breaker lacks any “memory” of the thermal history of the primary conductors and may not recognize that a thermal fault still persists quickly enough, compromising the integrity of the insulation on the primary conductors.
One approach uses a resistor-capacitor (RC) circuit that holds a voltage proportional to the temperature of the system. The RC circuit is charged during normal operation (i.e., operation above the thermal pick-up level). After the circuit breaker is tripped, the RC circuit discharges at a known rate. Another similar approach uses a timer circuit to estimate how much time passed from the time a circuit breaker was tripped to the time it was re-initialized. Upon re-initialization of the circuit breaker, the microcontroller will not immediately know the temperature of the primary conductor because it was powered down while the circuit breaker was tripped. To reestablish a thermal model, the microcontroller reads the voltage remaining on the RC circuit or the count on the timer circuit to estimate how much time passed and, assuming a rate of cooling, estimate the cooling of the primary conductor.
However, the RC approach and the timer approach suffer from a number of considerable disadvantages. Significantly, these approaches provide an estimation of the cooling of the primary conductor that is necessarily far less precise than the thermal modeling executed by a microcontroller. For example, the RC and timer approaches cannot take into account fluctuations in ambient temperature or other environmental factors that can affect the rate of cooling of a primary conductor. Additionally, after manufacture, the RC circuit or timer circuit cannot easily be modified to adjust for different operating conditions or protected loads. Further, RC circuits or timer circuits add cost and complexity to the system.